1. Technical Field of the Invention
The present invention relates to error correction methods, and more particularly, to a method for error correction using packet combining techniques during a soft handover process.
2. Description of Related Art
Digital cellular mobile systems normally apply a combination of forward and backward error correction techniques to improve the bit error rate of radio channel transmissions over the air interface. Forward error correction (FEC) involves a process of adding redundant information within transmitted bit streams at the transmitter that is used by the receiver to correct transmission errors.
The automatic repeat request (ARQ) technique is a form of backward error correction wherein the receiver requests the retransmission of packet data units (PDUs) that are incorrectly received from the transmitter. Packet combining techniques have been used within communication link protocols supporting automatic repeat request (ARQ). Data packets are transmitted and received sequentially within ARQ protocols. Once a packet is determined to be corrupt by the receiver, a retransmit request is sent back to the transmitter, requesting a retransmission of the corrupt packet. If the retransmitted packet is again determined to be corrupt by the receiver, the second packet may be combined with a stored version of the previously transmitted packet to correct any transmission errors. This technique is known as Type II Hybrid ARQ.
The problem with these techniques and others presently utilized for error correction is the necessity of including additional overhead (information) on the radio air interface in order to correct the transmitted information. For example, the FEC techniques require the addition of redundant bit information within the transmitted bit stream, and the ARQ techniques involve the transmission of additional messages requesting the retransmission of previously transmitted information. Thus, each of these techniques utilize additional bandwidth which may be at a premium in certain systems. Thus, a need has arisen for an improved error correction scheme that does not require the utilization of additional overhead within the radio air interface or a scheme which may be used to improve existing FEC and ARQ techniques.
The present invention overcomes the foregoing and other problems with a method of error correction during a soft handover process wherein a plurality of versions of a single data block are received at a radio network control node. At least two of the received versions of the data block are combined to create a substantially error corrected version of the data block for output to an attached MSC or other network node.
In a first embodiment, a first version and a second version from the received plurality of versions of the data block are exclusive or-ed together to determine bit positions within the data block where potentially erroneous bits are present. Each bit combination for the bits in the plurality of erroneous bit positions are determined, and the combination of bit values determined to be a substantially error corrected version of the data block selected based upon an error detection determination for each combination of bit values.
In a second embodiment, the plurality of versions of the data block are compared to determine which bit value occurs most frequently in each bit position of the data block. The bit values occurring a majority of the time for each bit position are selected, and the selected bit values are used to generate the substantially error corrected version of the data block.